Story Highlights

When running backs enter the NFL, it's as if they are stamped with a "use by date" that gives them the shelf life of yogurt.

Teams are always looking for younger, healthier and cheaper options. And they don't have trouble finding them.

Arizona Cardinals cornerback Patrick Peterson was a running back and defensive back in high school, but he made clear to college recruiters that he would play defense only in college.

Peterson knew the beating running backs take at high levels of the game, and he wanted to play as long as he could.

"The life span of an NFL running back is very short," Peterson said. "It was clear from Day 1, I was playing defense."

More than ever, running back has become a plug-and-play position. There are exceptions, such as the Minnesota Vikings' Adrian Peterson, but for the most part, teams want young players unscathed by the rigors of playing in the NFL.

That's why former Cardinals running Beanie Wells can't find a job, even though he's only 24.

Current Cardinals running back Rashard Mendenhall knows the feeling, too. He's 26 and was on his way to a third consecutive season of rushing for at least 1,000 yards for the Steelers when he suffered a knee injury in the final game of the 2011 season.

That injury contributed to an unproductive 2012 season, and the best offer he could get this off-season was a one-year contract from Arizona.

"That's part of it (playing in the NFL)," Mendenhall said. "You got so many guys coming out of college who can come in and get on pretty quick."

It's a trend that's developed in the NFL over the past 10 seasons. In the 2003 season, 34 running backs 30 or older had at least one rushing attempt, according to profootballreference.com.

The total has gradually dropped almost every year, down to 17 in 2012.

Pop Warner mamas might not want their babies growing up to be running backs, not if they have dreams of them having long NFL careers.

It's become a passing league, and only a handful of teams are building their offenses around the skills of a running back.

Cardinals coach Bruce Arians called Adrian Peterson "the last of the Mohicans, a guy who's going to tote it that many times, who you try to build your team around. Cleveland is trying it with Trent (Richardson), and Trent has a unique skill set, too. He's an excellent pass blocker, an excellent receiver."

What the Cardinals look for is a bigger back who can pass block, run the ball and catch.

The career span of a running back today reminds Mitchell of his time as a player, for the Cardinals (1981-89). Back then, backs were workhorses who received plenty of carries. But hits took a toll. In 1988, for instance, only 15 running backs 30 and older carried the ball in the NFL.

"Back then if you got to 28 (years old), you were doing great," Mitchell said.

It's that way now, too, but for at least one additional reason: money. The theory is that running backs can be found throughout the draft and in rookie free agency.

Why pay a veteran big money, especially when he has been beat up for five or so years, when a rookie, who is healthy, could give you the same production?

At a $2.5 million cost this year, Mendenhall is a bargain if he returns to form. If he doesn't, the Cardinals haven't wasted a ton of money.

The Cardinals selected two backs, Stepfan Taylor and Andre Ellington, in the fifth and sixth rounds. A fifth back, Alfonso Smith, has lasted three NFL seasons despite being a rookie free agent.

"What's interesting is when I started playing football, I thought I was a wide receiver or a cornerback," Smith said. "The reason I started getting recruited was cornerback, then they happened to see me at running back, and it all came from there.

"I tried to avoid it, but if it's God's plan, it's his plan."

Besides, it's not easy to give up playing running back. Carrying the ball is addictive and not all players are built to play elsewhere.

"If you're a guy with a ball in his hands, I think you're going to stay there," Mendenhall said, "unless you're Patrick Peterson and can do everything."

Cardinals running back Ryan Williams, a second-round pick in 2011, knows well the position's perils. He suffered a torn patellar tendon and missed his rookie year. A shoulder injury ended his 2012 season after five games. Another serious injury might result in unemployment.

Still, Williams has no regrets.

"I would do it all over again," he said. "I'm the happiest person in the world with a football in my hands, and playing football in general."